Transmitters in base stations and terminals for mobile telephony as well as transmitters for broadcast and other wireless systems all need power amplifiers (PA) to amplify the radio frequency (RF) signal to the antenna. Often, this PA needs to be very efficient to increase battery time, decrease the energy cost, and minimize cooling needs.
Traditional class B and AB PAs usually operate with constant load and constant supply voltage. The “optimal load” (Ropt) is the load that gives the highest output power under allowed operating conditions. Class B or AB operation means that the transistor current pulses approximately have the shape of half-wave rectified sine waves. The current pulses have largely the same shape at all amplitudes, and both the RF output current and the DC supply current, and hence DC power, are therefore approximately proportional to their height. This is also the case with the RF output voltage. The RF output power is proportional to the RF output current squared, which means that the DC to RF efficiency is approximately proportional to the RF output voltage amplitude. Due to this proportionality, the average efficiency for a class B amplifier outputting a signal whose average signal level is well below the maximum (peak) level is low compared to the efficiency at maximum output.
Dynamic Drain Bias (DDB) amplifiers increase the efficiency by reducing supply voltage according to the RF voltage needs of the amplifier. It uses a continuous RF-voltage-envelope-following supply voltage that minimizes the average voltage drop across the transistor. Dynamic drain biasing is the collective name for all similar techniques: Envelope Elimination and Restoration (EER), Envelope Tracking (ET), Drain (Bias) Modulation, Collector Modulation, Plate Modulation. Amplification of the envelope into a dynamic supply voltage is done in an efficient, switch mode or switch mode assisted, baseband amplifier. A DDB controlled amplifier is illustrated in FIG. 1.
The input signal (in baseband, IF, or RF form) 108 is fed to a drive/control signal processing block 100. This block outputs a drive signal to the RF amplifier 106 via an up converter 102, and a Vdd 112 (supply) voltage control signal to a switched mode power supply (SMPS) 104. The SMPS outputs one varying supply voltage that is fed to the amplifier 106.
Dynamic Drain Bias amplifiers are essentially neutral to series or shunt loss. Both these types of loss degrade efficiency equally at all output power levels. DDB controlled amplifiers are instead extra sensitive to residual voltage drop across the transistor at low output levels, an effect that is present for example in bipolar transistors. The efficiency curve of a DDB controlled amplifier with all three types of loss is shown in FIG. 2.